As the eaglets start to sprout feather cloaks, we’re getting questions about natal down, thermal down, and juvenile feathers. Unless otherwise stated, the information in this blog applies to altricial birds, although most research in this area has been done on precocial and semi-precocial birds like ducks, geese, and cranes. Altricial and precocial birds have some marked differences in pre-hatch follicular development and post-hatch molts.

Does thermal down sprout from natal down pores? Do flight feathers emerge from thermal down pores? When do eaglets really start growing flight feathers? Feather growth and development is a remarkably complicated subject. We’ll start by defining a few terms.

What we call natal down – the adorable fluffy white plumage that covers hatchling eaglets – is known as neoptile feathers to people who research feather development. All feather follicles develop in the egg, but in altricial birds like bald eagles, most follicles don’t activate prior to hatching. Those that do produce natal down, which is symmetrical, uniform, and general in purpose. In altricial birds like bald eagles, follicles that become active prior to hatch can only grow down.

What we call thermal down – the greyish plumage that begins to replace neoptile feathers at about eight days of age – is known as mesoptile feathers to the same people who call natal down neoptile feathers. While we don’t see mesoptile feathers right away, mesoptile feather germs ‘wake up’ and begin the work of assembling feathers shortly after hatch. As the name suggests, mesoptile feathers are intermediate feathers – more complicated than the natal down they replace and simpler than the juvenile feathers that will replace them. Nestling eaglets grow feathers three times – once in the shell and twice after hatch – and molt them twice. In altricial birds like bald eagles, follicles that become active after hatch can grow down or specialized feathers.

What we call juvenile feathers – the fully-formed feathers that begin to replace mesoptile feathers between roughly 20 and 28 days of age – are also known as juvenile feathers to people who research feathers, although any given species of bird may have more or fewer stages between natal plumage and juvenile feathers than the one stage that bald eagles have. Most juvenile feathers emerge from the same pores that produce mesoptile feathers, although it’s unclear whether all down feathers do.

Does thermal down sprout from natal down pores?For the most part, no – but it’s complicated. While an eaglet’s thermal down appears, well…downy, it includes both down and downy-looking precursors (pennaplumae) to wing, tail, contour, filoplume, and semi-plume feathers. We know that follicles with smaller diameters produce down and follicles with larger diameters produce all other feathers, although follicular size, shape, and complexity may vary in accordance with the size, shape, and complexity of the feathers they produce. So feathers don’t sprout from pores that produce down. Having said that, we don’t know if the small, shallow follicles that produce natal down die to make way for new structures (apoptosis), begin producing mesoptile down, or enter a resting phase in which they restructure themselves to grow juvenile or adult down.

In short: most mesoptile feathers don’t sprout from natal down pores. Complex feathers and their precursors grow from follicles that become active after hatch. But it is unclear whether follicles that become active before hatch die, switch to producing mesoptile down, or take a little R&R prior to producing juvenile, sub-adult, and adult down.

Pre-programmed death (apoptosis) and replacement make a lot of sense. But many species of birds produce multiple kinds of down. It might make sense for some post-hatch follicles to produce one kind of down and all pre-hatch follicles to produce another. I’m not sure which system would be more efficient.

Do flight feathers emerge from natal down pores? No. That we know, natal down pores can only produce down.

So the sequence is natal down, thermal down, juvenile feathers, right? You may have heard that nature abhors a vacuum. But nature really hates well-defined lines! Each plumage sequence emerges at a fairly predictable time, but neoptile down, mesoptile down, mesoptile pennaplumae, and emerging juvenile plumage can all be seen on an eaglet between roughly its third and fifth week of life. To make things even more complicated, neoptile down, mesoptile down, mesoptile pennaplumae, and emerging juvenile feathers may not be easily distinguishable from one another depending on their location, stage of development, and feather type.

Olga Fedorovna Chernova’s work on Cranes found that this mix of feathers assists thermoregulation and helps protect emerging juvenile plumage by surrounding it with softer neoptile and mesoptile feathers. In altricial birds, it might also help aid embryonic and hatchling development by deferring the cost of growing feathers.

So eaglets don’t grow tissue in the first half of nest life and flight feathers in the second half of nest life? It’s more complicated than that statement makes it sound. It takes a lot of energy, water, and protein to extrude a covert feather, and some of those feathers (think primaries, secondaries, and tail feathers) are very specialized. Shortly after an altricial bird hatches, its feather germs ‘wake up’ and begin assembling the building blocks they need to grow feathers. In that sense, feather growth starts right away! But feathers take a lot of resources and eaglets don’t start extruding their most resource intensive feathers until the third week. Juvenile feather growth starts as early as 20 days, but doesn’t really take off before about 35 days.

This turned out to be a much more complicated subject than I thought! Most research has been done on economically important precocial birds like chicken, ducks, and geese. Some species go through even more molts than eaglets, and molt sequences can be incredibly complicated. Researchers may be focusing on commercial concerns (how do we make ducks and geese grow more or fewer feathers?), evolutionary or embryological concerns (where did feathers come from and how do they develop?), health (studying how feathers develop can help us understand disorders of the integuement), or comparative analysis (skin versus feathers versus scales). These resources helped me understand and write about the issue: